Process for making latent dye salt image visible



United States Patent 3,510,300 PROCESS FOR MAKING LATENT DYE SALT IMAGE VISIBLE Richard A. Fotland, Lyndhurst, and Harry L. Fichter, Jr., Lakewood, Ohio, assignors to Horizons Incorporated, a division of Horizons Research Incorporated, a corporation of Ohio No Drawing. Filed May 24, 1966, Ser. No. 552,414 Int. Cl. G03c 5/04, 9/00, 1/52 US. Cl. 96-27 9 Claims ABSTRAQT OF THE DISCLOSURE It is already known that visible useful images can be formed from the initial latent silver image in silver halide photography utilizing chemical development in order to effect a tremendous amplification of the latent image resulting from photographic exposure of a silver halide film. It is believed that, as a result of such exposure, an initial image is formed which consists of specks of silver metal and that such specks are capable of catalyzing the reduction of an entire silver halide grain. Such silver halide films and papers require wet processing and in addition they are inherently limited as to their resolution due to the graininess of the silver halide emulsion. Furthermore, with silver halide materials, the speed of a given system is fixed by the grain size since the wet processing development requires the full development of any grain.

Films and papers of the diazo or photochromic type do not require wet processing and are capable of providing very high resolution imagery. Such systems function only with quantum yields lower than unity; quantum yield being defined here as the ratio of the number of image molecules formed to the number of incident exposing photons. Thus, these materials are inherently of low sensitivity.

It is an object of the present invention to provide a process for developing latent images in molecular dye films and papers which requires no wet processing and which retains the high resolving power inherent with molecular image formation and which attains speeds equal to that of wet developed, fine grain, high resolution, silver halide materials.

Another object of the invention is to provide a process in which the speed of the photosensitive system is controlled by the optical development exposure, i.e., with a low optical development exposure a low amplification may be realized while with greater optical development exposure a greater amplification may be obtained.

These and other objects will be made apparent from the description which follows in which a preferred embodiment of this invention is set forth by way of illustration.

In the description which follows it is to be understood that the process is not dependent on the manner in which the initial invisible, latent, faint or barely visible dye image is formed.

The manner in which the initial image is produced may depend in'part on the specific chemical compounds involved and in part on the specific manner in which imagewise exposure is effected. For example, the image creating exposure may be to radiation in the ultraviolet radiation, in the visible, X-radiation, electron beam radiation, or the image may be created by electrolysis creating traces of dye by passage of an electric current, or it may be created by a spark discharge or in any other manner suitable for the material utilized.

While not wishing to be bound by any theoretical explanation, it is believed that, in the present invention, the initial (trace) image consists of an infinitesimal number of dye molecules, each of which is capable of absorbing radiation in a very narrow band of the spectrum and that intensification is the result of the action of such radiation on the latent image.

For purposes of illustration, the following generalized example is presented. The film utilized contains at least one dye salt progenitor, P, and at least one activator, A, preferably dissolved in a solution containing a suitable film-forming plastic and formulated and cast into a film supported upon either a paper, cloth or synthetic resin base as described in the several patents noted below. Upon initial exposure, whether to an electron beam, X-ray beam, or ultraviolet or visible light source of radiation, a salt, S, of P is formed in the exposed areas of the film by a photochemical reaction involving both P and A. For low exposures, the quantity of S formed by the initial exposure might only range from 10 to 10,000 parts per million parts of A available. For the process of optical development according to the present invention it is necessary that the optical absorption spectrum of S differ from the absorption spectrum of A, P, or any complex formed between A and P. After initial imagewise exposure, optical development is carried out by illuminating the entire film with radiation from that portion of the spectrum comprising the region of optical absorption of S. Image areas in which the salt S is present absorb the developing radiation while areas containing only A and P and which were not initially exposed, i.e., nonimage areas, do not absorb the optical development radiation. It will be apparent that the development radiation must not contain substantial amounts of radiation which will be absorbed by A, P, or any complex formed between A and P since this would cause fogging in the nonimage areas.

It appears that during the process of optical development the energy of radiation absorbed by S is transferred to either A, P, or to a complex between A and P in such a manner that additional salt, S, is formed. It also appears that the developing radiation absorbed by the salt, S, raises the salt to a high excitation energy level and that this energy may then be transmitted to additional starting compounds, A, P or to the complex between A and P, if one is present, with consequent conversion of additional amounts of starting compound to form additional salt, S. As development proceeds, the amount of salt, S, is continually increasing and since the rate of development is proportional to the amount of salt, S, the rate of development increases in a constant fashion with development time while the total development multiplication or amplification varies exponentially with development time.

The development process may be stopped at any time, by simply cutting off or terminating the blanket exposure to the developing radiation thereby providing control over the degree of image amplification or the overall quantum gain or sensitivity of the system.

The process is therefore photocatalytic in nature and the development multiplication proceeds until all of the starting materials are used up or the developing radiation is switched off. Tremendously high sensitivities may be obtained by virtue of the autocatalytic nature of the process and it has been found that the number of image mole- 3 cules and hence the image density may be increased by a factor of 1,000 to 100,000 by the described technique of optical development using suitably selected development using suitably selected development radiation.

A principal limiting factor appears to be the buildup of fog in nonimage areas, and it is believed that this background fog is due in part to the presence of minute amounts of impurities present in the nonimage areas, which impurities absorb the optical development radiation and contribute to the formation of salt in the same manner as salt is formed in the exposed (image) areas. The use of suitably purified materials when formulating the film tends to minimize the effect of this source of fogging.

Among the nonsilver photosensitive compositions which are susceptible to the optical development described above are the free radical photosensitive compositions patented by Eugene Wainer and his coworkers, including those described in the following United States Patents and copending patent applications:

PATENTS APPLICATIONS 270,551, now Patent 3,285,744 273,569, now Patent 3,272,635 309,407, now Patent 3,284,407 395,903, now Patent 3,342,595 398,097, now Patent 3,342,602 399,859, now Patent 3,342,603 399,892, now Patent 3,342,604 485,535

487,398, now Patent 3,420,532 500,321, now Patent 3,445,232 502,498, now Patent 3,443,945

The following specific examples are intended to demonstrate the wide variety of application and formulations which may be optically developed in accordance with this invention and are not intended to be construed as limiting the invention.

EXAMPLE 1 A photosensitive surface was prepared under a red safelight by dissolving 1.4 grams of carbon tetrabromide and 50 milligrams of 4-paradimethylaminostyryl quinoline in 4 ccs. of a 10% solution of polystyrene in benzene. The solution was coated on a 5 mil-polyester film base using a Bird applicator bar having a wet coating thickness of 1.5 mils. The film was allowed to dry three minutes and then exposed for one second on an Eastman Model 101 process sensitometer. The coating procedure is fully described in Photographic Science and Engineering, volume 8, No. 2, pages 95-103 1964) and volume 5, page 98 (1961). Immediately after imagewise exposure, no image was visible, and the film was then developed optically by exposure to an intense red light source for one minute. The red light source consisted of two 8-inch long 1.5 kw. tungsten iodine lamps spaced parallel and two inches apart. Unwanted light was filtered out of the enclosed light source box using a Corning #2030 red filter having a sharp cutoff at 660 millimicrons. The infrared radiation was filtered out by a one centimeter path-length water cell. During optical development the film temperature did not rise more than 4 to 5 C. The film to lamp spacing during optical development was twelve inches. The film was then fixed by a 30-second rinse in a solvent bath consisting of 4 parts petroleum ether and one part acetone. The exposure index (American Standard speed) obtained was between about 0.01 to 0.1. Higher speeds were obtained when the CBr, has been purified by vacuum sublimation and the 4-paradimethylaminostyryl quinoline has been purified by chromatography. Dried reagent grade benzene was employed to prepare the polystyrene solution.

EXAMPLE 2 Optical development of a similar image was obtained by repeating Example 1 using 1.0 gram of pentabromoethane in place of the carbon tetrabromide and mg. of Leuco Crystal Violet in place of the styryl quinoline.

EXAMPLE 3 One gram of Leuco Opal Blue and 2.4 grams of iodoform were dissolved in 30 cc. of a 6% solution of polyvinylchloride in tetrahydrofuran. The solution was coated with a wet thickness of three mils on five-mil Mylar polyester film and allowed to dry. A series of exposures were made using monochromatic illumination to determine the energy sensitivity of this film. At 400 millimicrons an exposure of one mw.-sec./cm. generates an image density of 0.5 density units and the image color is blue. At 430 millimicrons the sensitivity has fallen to one-half its peak value at 400 millimicrons. The film is thus sensitive to the near UV and violet portions of the spectrum. The film processing is carried out by exposing the film to the red light source described in Example 1 for a period of one-half to one minute. In this example, however, the water bath IR filter is removed and thus the film temperature is allowed to rise during development to approximately C. The film is fixed as well as developed during this processing. The shelf-life of this film is in excess of two months and because of its high speed and simple processing possesses obvious advantages as a recording medium when exposed with a UV mercury source or a cathode ray tube employing a P-16 phosphor.

EXAMPLE 4 An electron beam sensitive organic film was prepared by dissolving 100 milligrams of carbon tetrabromide, 200 milligrams of Leuco Crystal Violet, and 500 milligrams of hexachloroethane in 14 cc. of a 10% solution of polystyrene dissolved in toluene. The film was prepared by coating this solution onto the surface of five-mil Mylar polyester as described in Example 1, using a wet coating thickness of 1.5 mils. The film was exposed with an electron beam having an accelerating potential of 20 kilovolts. After exposure, no image was visible and the film was developed by exposure to the red light source decribed in Example 1; once again omitting the water cell. Development times as short as 10 to 20 seconds are sufli cient to bring out a high density blue image. The development time and degree of development are limited by the buildup of fog in nonimage areas. At a charge exposure of 10" coulombs per sq. cm., image densities in the range of 1.5 to 2.2 are observed. The maximum density obtainable in this material is 2.3. This film is sufiiciently insensitive to visible so that, previous to electron beam exposure and development, it may be handled for several minutes in ordinary fluorescent illumination levels of 50 ft.-candles. After electron beam exposure and development it may be handled under ordinary fluorescent illumination for several hours without fogging. The film may be subsequently heat fixed by exposing the film for a period of several seconds to a source of radiant energy of sufiicient intensity to raise the temperature of the film to about C. The shelf life of this material is fairly good; the maximum speed dropping by a factor of approximately 2 after a storage period, at room temperature, of 3 months.

EXAMPLE 5 An X-ray sensitive film was prepared by employing the formulation in Example 4 with the exception that the concentration of the polystyrene in the toluene was increased to 20% and a wet coating thickness of 6 mils was employed. The higher concentration and greater coating thickness were employed in order to increase the X-ray absorption in the films. This film was exposed through a foil step tablet to the X-ray beam from a General Electric XRD X-ray difiration apparatus. Similar exposures were made using Kodak no screen medical X-ray film. The X-ray sensitive organic film was developed in the same manner as that described in Example 4 and the medical X-ray film was developed according to the manufacturers instructions. It was found that the organic film possessed 1 the sensitivity of the medical X-rayfil m. Although this sensitivity is quite low, because of the high resolution of the organic film and the simplicity of processing, certain applications where long X-ray exposures may be employed, such as in X-ray microscopy, could employ this organic film.

EXAMPLE 6 An optically-developed electrolytic-type electro-sensitive paper was prepared by dissolving milligrams of Leuco Opal Blue and 100 milligrams of potassium bromide in a solution consisting of 1 cc. of tetrahydrofuran, 2 ccs. of ethylene glycol, and 2 ccs. of a 10% solution of polyvinyl pyrrolidone dissolved in methyl alcohol. The paper was prepared by merely dipping the paper into this solution. After removing the paper from the solution, the paper was allowed to dry for several minutes. The writing speed was determined on a facsimile reproduction apparatus. The optical development was carried out as described in Examples 3 to 5. At an exposure level of 2 milliamps, using a IO-mil diameter tungsten wire stylus and a recording potential of 300 volts, a dense image was obtained (after development) at a writing speed of l00"/ second. The sensitivity is approximately 50 times faster than conventional electrosensitive electrolytic type recording papers where no post-exposure development technique is employed. This example illustrates the usefulness of optical image development in situations in which the initial trace dye image is formed in a manner other than by exposure to various sources of radiant energy.

EXAMPLE 7 The electrosensitive paper described in Example 6 was employed in conjunction with a photoconductive layer in a light sensitive image-forming system. This photoconductive layer may be formed by a number of techniques well known to those skilled in the art. Such photoconductive films have been described by Thomson and Bube 1 as well as by Nicoll and Kazan. Commercially available activated cadmium sulfide powder was mixed with a polystyrene dope and diluted with toluene in the ratio of 4 ccs. of polystyrene, 2 grams of cadmium sulfide, 2 ccs. of toluene. This viscous liquid was spread with a coating blade onto a conducting glass substrate and allowed to dry. After the conductive layer had dried, a sheet of electrosensitive paper, prepared. as described in Example 6, was laid directly upon the photoconductive surface. A conducting rubber pad was laid over the electrosensitive paper and the sandwich was excited with a potential of 600 volts applied between the conducting Nesa glass and the conducting rubber. Sufficient pressure was applied upon the conducting rubber pad to provide an intimate contact between the electrosensitive paper and the photoconductive film. An image was projected, employing a conventional projector, onto the photoconductive surface through the transparent conducting glass. The highlight intensity of the imagewas 500 ft-candles and the exposure time was 1 second. The sandwich was disassembled and the electrosensitive paper processed Thomson, S. M. and Bube, R. H.: High-sensitivity photoconductor layers,

Rev. Scientific Instruments, 26, 664-665 1955).

6 by red-light development as described in Example 6. The image-struck areas in the photoconductor permitted an electrical current to flow through the electrosensitive paper in this area, forming a latent image. After redlight development, an intense blue image was formed in these light-struck areas.

Although the optical development of visible images has been successfully practiced on latent images in films formulated with a variety of dye bases including merocyanines, merodicarbocyanines, cyanines, carbocyanines, dicarbocyanines, tricarbocyanines, azacyanines, merocarbocyanines, azocarbocyanines, styryls, butadienyls, azostyryls, hexatrienyls and leuco compounds, such as leuco diphenylmethanes, leuco triphenylmethanes, xanthenes, selenoxanthenes, thioxanthenes, thiazines, oxazines, fluorenes, rhodamines, and phenoxazines, in compositions such as those described in the above noted patents, the present invention is not to be confused with the use of heat to intensify visible images as described in several of the above noted patents.

A majority of the photosensitive compositions described in the above noted patents are merely fogged or only exhibit very slight image intensification when exposed to heat without the presence of visible radiation, whereas the same composition, exposed to filtered visible light, is successfully developed without noticeable fogging.

What is claimed is: ;1. A process for producing a visible image in a radiation sensitive medium solely as a result of exposing said medium to radiation of suitable wavelengths, which comprises:

preparing a radiation sensitive medium containing an organic halogen compound in which at least three halogen atoms are attached to a single carbon atom and containing a dye salt progenitor compound;

exposing said medium to a pattern of radiation to which said medium is sensitive, in an amount sufiicient to produce a latent image of said pattern of radiation as a result of a radiation induced reaction between said organic halogen compound and said dye salt progenitor, whereby a dye salt is produced in the exposed areas of said medium, the amount of radiation in the exposed areas of said medium being insufficient to produce a visible image in said areas; and

thereafter exposing all of said medium to a dose of radiation which lies within the optical absorption band of at least one compound constituting a substantial component of the latent image in said medium, but which lies outside of the optical absorption bands of said organic halogen compound and said dye salt progenitor compound, whereby said latent image is converted to a visible image in said medium Without any noticeable change in the portions of said medium not exposed to said initial pattern of radiation.

2. The process of claim 1 wherein the active ingredients are supported in a sheet of paper to which they are applied as solutions and by which the solutions have been imbibed andare thereafter dried.

3. The process of claim 1 wherein the temperature of said layer is maintained and prevented from rising substantially during said second exposure.

4. The process of claim 1 wherein the radiation utilized for development of a visible image from said latent image lies between 5700 A. and about 7900 A.

5. The process of claim 1 wherein the said medium comprises active ingredients supported in a binder of film forming plastic, in which they are dissolved.

6. The process of claim 1 including the step of controlling the temperature of said medium so that development of a visible image and fixing of same occur simultaneously.

7. The process of claim 1 in which the first dose of radiation is in the near UV and violet portions of the spectrum and the second dose of radiation is in the red and orange portions of the spectrum.

8. The process of claim 1 wherein the first dose of radiation is from an electron beam source and the second dose of radiation is visible light.

9. The process of claim -1 wherein the first dose of radiation is X-ray.

References Cited UNITED STATES PATENTS 3,042,519 7/1962 Wainer 96-90 3,121,633 2/1964 Sprague et a1. 9648 3,143,940 8/1964 Brown et a1. 96-48 XR' 8 3,147,117 9/19'64 .Wainer et a1 96-48 3,152,902 10/1964 Jacobs 96-48 XR OTHER REFERENCES 5 I Kosar, 1.: Light-Sensitive Systems (previously of record), pp. 367, 368 and 397 added to the record.

August 1965. John Wiley & Sons, Inc., New Yo DAVID KLEIN, Primary Examiner U.S. Cl. X.R.

rk, p. 

